A conventional window shade generally is lifted or lowered through a rope. The rope usually is exposed outside and could cause a hazardous condition by tangling or strangling children playing nearby. Hence for safety reason the rope of the window shade is generally collected in a winding device located on the bottom rail of the window shade without exposing to prevent risks.
There are many types of pulling cord winding devices for window shades on the market. FIG. 1 illustrates one of the examples (namely U.S. Pat. No. 6,675,861 B2). It has a pulling cord winding device A installed on a front end of a bottom rail B1. The device includes a cord braking board A1 which has an upper pressing member A10 and a lower pressing member A11 which is coupled with a collar A12 to clamp a pulling cord C. When a window shade B is extended or retracted, the upper pressing member A10 is pushed downwards by a spring A13 to compress the pulling cord C. The compression surface A101 of the upper pressing member A10 and an opening A120 of the collar A12 form a V-shape shearing force (referring to FIG. 2A). The pulling cord C tends to be severed by the shearing force. When the lower pressing member A11 is moved upwards, the compression surface A111 of the lower pressing member A11 and a bore A100 of the cord braking board A1 form an inverted V-shape shearing force. This also tends to sever the pulling cord C (referring to FIG. 2B). Hence the pulling cord C is difficult to be anchored on a desired location. The aforesaid pulling cord winding device A not only is troublesome during operation, but also cannot anchor the window shade on a desired location during extension and retraction. Moreover, the pulling cord winding device A consists of many elements, and is too bulky. A bigger bottom rail B1 is needed to hold the pulling cord winding device A. In short, the entire device takes too much space and is too costly. The enlarged bottom rail B1 also becomes too heavy. And the spring A13 installed inside the pulling cord winding device A has to withstand a greater force. This shortens the service life of the device.